1. Field of the Invention
The present invention relates to clamping differential drivers. More particularly, the present invention relates to the field of digital data transmission. More particularly yet, the present invention relates to improving the consistency of digital data transmissions over long cables and especially to reducing pattern-dependent jitter when such cables are used. Still more particularly, the present invention is a clamping circuit for reducing pattern-dependent jitter in digital data transmission/reception by means of amplitude clamping and also for high-frequency filtering in such transmission/reception.
2. Description of the Prior Art
Digital data is typically transmitted between electronic devices such as telephones, facsimile machines, computers, and the like, by simple electric cables. Such a cable can be as simple as a pair of twisted wires, the impedance of which is about 50-100 ohms, an acceptable and accounted-for impedance. However, when such cables are used to transmit data over great distances (which can be defined as distances in excess of 30 meters) and/or when the rate of data transmission becomes high, the cable itself becomes an important component of the overall system, in that its increasing resistance can result in data errors being introduced at the receiving end of the cable, related to an increase in the ramping of bit patterns.
For present purposes, it is sufficient to think of the transmission cable as a single pair of wires connected to a transmitter which at any instant is imposing on one member of the pair a signal and on the other member of the pair a signal-complement. The signal is binary and at any time one wire will have imposed on it by the transmitter either a HIGH signal or a LOW signal, while the complement of that signal is imposed on the other member of the pair. Of course, either wire can conduct either signal. The two wires are typically connected to a differential receiver for subsequent processing.
Under ideal transmission/reception conditions, the signals imposed at the transmission end of the line will be exactly reproduced at the receiver. This will be achieved if the rise-time and fall-time (for LOW-to-HIGH and HIGH-to-LOW transitions, respectively) of the voltage on the cable is extremely well defined and consistent. Although this is physically impossible, given that the cable is a real physical entity with its own characteristic impedance leading to finite rise- and fall-times, it is sufficient as a practical matter that these times be short with respect to the duration of the individual pulses making up the digital signal.
As transmission lines become longer--with a concomitant increase in their rise--time-and signal rates higher--with a concomitant decrease in pulse-width--it becomes increasingly more challenging to ensure that the signal seen at the receiver bears a unique correlation to the signal imposed at the transmitter end of the cable. Although it is not necessary that there be exact identity between these two signals, it is necessary that the correlation does not depend on things such as a particular pattern of HIGHs and LOWs transmitted.
For definiteness, this discussion will address most transmission systems, with varying definitions of what constitutes HIGH and LOW, respectively. In particular, it will be directed to systems in which a "high" voltage level is identified generically as VOH, while a "low" ,voltage level is identified generically as VOL. The particular voltage levels and swings associated with VOH and VOL depend upon the particular transmission/reception circuitry associated with the transmissions. For example, in emitter coupled logic (ECL) systems and low voltage differential systems (LVDS), the swings between VOH and VOL are relatively small--on the order of 700 mV for example. On the other hand, for transistor-transistor logic (TTL) systems, the swings between VOH and VOL may be much greater--on the order of 2 V or more.
In order to have a reproducible transmission of data, there must be a correlation between the HIGH/LOW or LOW/HIGH "cross-over" , at the transmitter and the cross-over at the receiver. If that correspondence is not achieved, an incoming signal may not be translated properly and bits of information may be lost for failure to be read properly. Although the respective cross-overs at the two ends of the cable will not occur at precisely the same instant, it is important to ensure that any delay that exists remains constant and, in particular, not dependent on the pattern of HIGHs and LOWs imposed by the transmitter.
FIG. 1 depicts a simplified transmission/reception system in accord with the above discussion. It shows an incoming signal 5 giving rise at the input driver 6 to itself and to its complement prior to being imposed on the transmitter-ends of a signal line 8 and a signal-complement line 9, the receiver ends of which are connected to an input driver or translator 7, generally a differential driver. By this circuit arrangement, HIGH/LOW signal strings representing digitized information--such as voices, music, pictures, etc.--are delivered from one location to another. It is the transmission fidelity of these strings that is of interest here. The particular problem addressed is data loss arising from transition time jitter which in turn is due to HIGH-to-LOW transition-interval variation based on signal string pattern.
If the charge time of the transmission line is significant with respect to the pulse-width of the signal imposed on the line by the transmitter, then the amplitude of the HIGH pulse seen by the receiver will vary with signal pattern. To see this, consider first that the signal imposed by the transmitter consists of a simple string of pulses 01010101010101--the highest transmission frequency possible. When the rise time of the transmission line is greater than the pulse width, the voltage seen by the receiver during a single HIGH pulse will not reach the maximum VOH; indeed, the voltage at the receiver will still be rising when the HIGH-to-LOW transition is imposed. In the extreme case, the transmission cable may load down the transmitter so much that the voltage seen at the receiver may never reach an unambiguous HIGH level associated with the particular VOH of the particular transmission/reception system. This is not the situation being addressed here, but rather the situation where the voltage at the receiver reaches some level higher than a minimal VOH but lower than an ideal VOH during a time interval of one pulse-width. Thus, even though the voltage at the receiver does not reach the level imposed by the transmitter, it will be high enough to cause a LOW-to-HIGH transition at the receiver end. It can be seen that, up to a point, the more HIGH pulses are transmitted in succession the higher the voltage will reach at the receiver. This per se will not cause a problem, since anything above the minimal VOH will be read as HIGH. The problem comes from the fact that the higher the transmission-cable voltage is when a HIGH-to-LOW is imposed, the longer it will take for the voltage on the transmission line to fall to the cross-over voltage, that is, to the voltage where the receiver will make the transition to LOW. Thus, for example, the mid-string HIGH-to-LOW transition at the receiver will occur at a different time for the string 11110000 than it will for the string 01010000. Since generally the systems will be driven by an internal system clock, this means that, depending on the particular nature of the signal string being transmitted, the instant at which a particular HIGH-to-LOW transition occurs will deviate from the clock transition time by an amount depending on the preceding signal string. This shifting back and forth about the clock period is referred to as pattern-dependent jitter, something that may result in an indeterminant signal level, resulting in the dropping of a bit. While loss of a single bit is of minimal concern, it is undesirable to have unnecessarily high bit error rates in transmissions.
As far as is known, there is no prior art relating to the reduction of the pattern-dependent jitter described above. As is discussed below, the approach of the present invention to this reduction involves a diode-based clamping of the receiver-input voltage. In the general field of using diodes to clip amplifier input, for example for protection against electrostatic spikes, there is of course prior art. For example, U.S. Pat. No. 5,589,813 issued to Nielsen in 1996, describes the use of a pair of diodes essentially as electrostatic discharge (ESD) protection devices in a communications system. Nielsen thus provides a pathway for voltage transients that may or may not ever occur. This contrasts greatly with the use of diodes to address pattern-dependent jitter on transmission lines that otherwise would occur on a fairly regular basis. Moreover, Nielsen includes a capacitor in series with the diodes, which capacitor will cause an opening of the circuit including the diode pathways after there is a transition between HIGH and LOW signals, thus preventing further signal transfer through the open pathway.
What is needed is a circuit that actively clamps excess amplitude voltage signals particularly evident of pattern-dependent jitter. That is, what is needed is a circuit that limits the relative maximum amplitude between the complementary signals delivered by the differential pair of transmission lines. What is also needed is such a circuit that remains operative during signal transitions and during fixed patterns of either HIGH or LOW transmissions. Further, it is also preferred to have a circuit that provides some beneficial high-frequency filtering in addition to clamping. Still further, what is needed is such circuitry that is readily compatible with existing differential drivers and that is relatively simple to manufacture.